1. Field of the Invention
The present invention relates to magnetic circuits and electrodynamic loudspeakers using the same, and more particularly, the present invention relates to a loud speaker having a two-gap, one-voice-coil structure in which a diaphragm has a widened linear amplitude region by providing magnetic gaps at two positions in the axial direction of a magnetic circuit and disposing a voice coil in these magnetic gaps.
2. Description of the Related Art
In known electrodynamic loudspeakers in which a voice coil is placed in a magnetic gap provided halfway through the magnetic path of a magnet and a diaphragm is vibrated by feeding a signal current (voice current) to the voice coil, because sounds are distorted when the amplitudes of the voice coil and the diaphragm do not change linearly in accordance with the level of the signal current, it is required to maintain a constant magnetic flux, which acts on the vibrating voice coil, in the magnetic gap. However, as shown in FIG. 3, in a known electrodynamic loudspeaker, a pole piece 2 of a yoke 1 and an upper plate 3, forming the magnetic path of a magnet 4 magnetized in its axial direction, are opposed to each other via a magnetic gap G; a voice coil 5 having a winding width, i.e., a height, smaller than the width of the inner circumferential surface of the upper plate 3, is disposed in the magnetic gap G; and when the voice coil 5 is displaced upward or downward in the figure partially outside the magnetic gap G, the magnetic flux acting on the voice coil 5 sharply decreases. Therefore, the amplitude region of the voice coil 5 must be set small so as to suppress sound distortion. That is, the known electrodynamic loudspeaker has a problem in that turning the volume high causes sound distortion. Although it is possible to widen the linear amplitude region by making the winding width of the voice coil 5 greater than the width of the inner circumferential surface of the upper plate 3, the voice coil 5 becomes heavier in this case, resulting in a lowered electroacoustic transducing efficiency.
To solve the foregoing problems, as shown in FIG. 4, another known electrodynamic loudspeaker has been proposed in which a lower magnetic gap G1 and an upper magnetic gap G2 are provided at two separate positions in the axial direction (in the vertical direction in the figure), and the voice coil 5 is disposed in these magnetic gaps G1 and G2. A magnetic circuit of the known loudspeaker shown in FIG. 4 is characterized by the shape of the upper plate 3 placed on the magnet 4. More particularly, since the upper plate 3 has an annular indented groove 3a formed on the inner circumferential surface thereof which opposes the pole piece 2 of the yoke 1, the lower and upper magnetic gaps G1 and G2 are respectively formed below and above the indented groove 3a and between the upper plate 3 and the pole piece 2. Since the annular magnet 4 is magnetized in its axial direction so as to have an N-pole on its upper surface and an S-pole on its lower surface, the magnetic flux of the magnet 4 supplied to the upper plate 3 passes through the lower magnetic gap G1 and the upper magnetic gap G2 toward the pole piece 2, and then the magnetic flux flowing down in the pole piece 2 returns to the magnet 4 through a bottom plate 6 of the yoke 1. The voice coil 5, wound around a cylindrical bobbin 7, is vertically placed so as to partially oppose the upper portion of the lower magnetic gap G1 and the lower portion of the upper magnetic gap G2 when no current is fed. The top of the bobbin 7 in the figure is bonded to the inner circumference of a conical diaphragm 8 made from cone paper or the like. Also, the upper plate 3 has a frame 10 fixed thereon with screws 9, and the frame 10 supports the outer circumference of the diaphragm 8 through an elastic edge 11 and also supports the bobbin 7 through a damper 12 in a vibratable manner.
In the known loudspeaker having the above-described structure, even when the voice coil 5 is displaced so as to oppose the lower portion of the lower magnetic gap G1 or the upper portion of the upper magnetic gap G2 when a current is being fed, since the magnetic flux of the magnetic circuit, acting on the voice coil 5, can be maintained substantially constant, the linear amplitude region of the diaphragm 8 can be widened without making the voice coil 5 heavier in an unwanted manner, whereby the features of the loudspeaker can be improved. Such a conventional loudspeaker is disclosed in Japanese Unexamined Patent Application Publication No. 4-183200, for example.
Although the loudspeaker including the magnetic circuit shown in FIG. 4 more effectively widens the linear amplitude region of the diaphragm 8 than the loudspeaker including the magnetic circuit shown in FIG. 3, the former requires a very complicated cutting process for forming the annular indented groove 3a on the inner circumference of the upper plate 3, thereby leading to an increased manufacturing cost and machining precision. Also, even in the small-type loudspeaker, because the required lengths, i.e., depths, of the magnetic gaps G1 and G2 and the required gap between the magnetic gaps G1 and G2 provided in the axial direction are at least about 4 mm, the required thickness of the upper plate 3 is at least 12 mm, thereby creating a limiting factor in making the overall loudspeaker thinner. As a result, when the height of the magnetic circuit of the loudspeaker is limited, a magnet 4 having a reduced thickness must be used, whereby desired features of the loudspeaker are not likely to be obtained due to an insufficient amount of magnetic flux. In addition, because a plurality of screws 9 are needed to securely fix the frame 10 onto the upper plate 3 having a thickness of at least 12 mm, a plurality of screwing operations are required in the assembly process, thereby resulting in inefficiency.
The present invention has been made in view of the foregoing problems of the related art. Accordingly, it is an object of the present invention to provide a thin magnetic circuit and a thin loudspeaker by providing magnetic gaps at two axially separated positions without applying a complicated machining process on a component serving as a part of the magnetic circuit.
A magnetic circuit according to the present invention comprises an annular magnet magnetized in the axial direction of the magnetic circuit; an annular stacked plate disposed on the magnet and including first and second annular plates; and a yoke opposing the inner circumferential surface of the stacked plate across a cylindrical space. Upper and lower magnetic gaps are provided at two axially separated positions in the cylindrical space. Also, the inner circumferential portions of the first and second annular plates are deformed downward and upward, respectively, in the axial direction such that the inner circumferential surface of the stacked plate opposes the lower and upper magnetic gaps. In addition, the remaining portions of the first and second annular plates, extending outward from the deformed inner circumferential portions in the radial direction of the magnetic circuit, are stacked on the magnet.
A loudspeaker according to the present invention includes a magnetic circuit comprising: an annular magnet magnetized in the axial direction of the magnetic circuit; an annular stacked plate disposed on the magnet and including first and second annular plates; and a yoke opposing the inner circumferential surface of the stacked plate across a cylindrical space. Upper and lower magnetic gaps are provided at two axially separated positions in the cylindrical space. The loudspeaker further comprises a voice coil placed in the upper and lower magnetic gaps; a diaphragm connected to the voice coil; and a frame fixed to the stacked plate and supporting the diaphragm in a vibratable manner. The inner circumferential portions of the first and second annular plates are deformed downward and upward, respectively, in the axial direction such that the inner circumferential surface of the stacked plate opposes the lower and upper magnetic gaps. Also, the remaining portions of the first and second annular plates, extending outward from the deformed inner circumferential portions in the radial direction of the magnetic circuit, are stacked on the magnet.
In the magnetic circuit and the loudspeaker having the above-described structures, since the lower and upper magnetic gaps are formed at respectively predetermined positions simply by disposing the first and second annular plates on the magnet in a stacked manner and the annular plates require no complicated machining process, the linear amplitude regions of the voice coil and the diaphragm can be widened at low cost. Preferably, the inner circumferential portions of the first and second annular plates are deformed by pressing them by a predetermined amount in a predetermined direction. Also, in this magnetic circuit, by stacking these annular plates such that the inner circumferential portions of the annular plates oppose each other in an upside-down opposing manner, a predetermined axial gap is obtained between these inner circumferential portions, i.e., between the magnetic gaps. Accordingly, portions of the annular plates, which radially extend from the inner circumferential portions and which are stacked on the magnet, have a thickness equivalent to that of the two annular plates which are stacked together without leaving a clearance therebetween. As a result, the entire magnetic circuit and the loudspeaker can be made thinner, or their operational features can be improved by making the magnet thicker.
Also, with the above structure, the first and second annular plates can be standardized as a common component and assembled in the magnetic circuit in a mutually upside-down stacking manner. This results in no increase in the number of component types, provides easy component control, and reduces the component cost.